I've found out some information:
In 127u1 they move the on screen display to slider controls
In 127u6 they remove the use of the ~ key
In 128u5 they restore the master volume to the ~ key
I'e downloaded the 128u5 diff, trying to guess the lines where they restore the master volume and I've found this for ui.c
/* if the on-screen display isn't up and the user has toggled it, turn it on */
if ((machine->debug_flags & DEBUG_FLAG_ENABLED) == 0 && ui_input_pressed(machine, IPT_UI_ON_SCREEN_DISPLAY))
return ui_set_handler(ui_slider_ui_handler, 1);
If you edit 134 source, you can indeed see those lines in ui.c. I wonder what happens if I change ui_slider_ui_handler, 1 with a different number. I can't wait to arrive home to try it.